Net loss rates and distribution of molecular hydrogen (H2) in mid-latitude coastal waters

We present the first reported net loss-rate constants of molecular hydrogen, H2, in seawater. Net loss rates and depth profiles of hydrogen were measured in coastal seawater at two mid-latitude sites in eastern Canada: the St. Lawrence Estuary and Halifax Harbour, between November 2005 and July 2006. Net loss-rate constants ranged between 0.29 d− 1 and 6.07 d− 1 in the St. Lawrence Estuary, and from 0.14 d− 1 to 8.67 d− 1 in Halifax Harbour. The 0.2 μm−5 μm particle size fraction was associated with H2 loss, implying bacterial consumption. There was a correlation between net loss-rate constants for H2 and CO suggesting that both substrates shared a common sink. Dissolved hydrogen profiles measured in the St. Lawrence Estuary and Bedford Basin during winter and spring generally showed H2 levels below atmospheric equilibrium in surface waters and declining with increasing depth. Bedford Basin surface water was supersaturated (125–257%) on sunny days in June and July suggesting that net H2 production was linked to irradiance, although the exact nature of the source is yet to be determined. A series of light and dark incubation experiments showed lower net H2 uptake rates in irradiated samples suggesting either H2 photo production or light inhibition of H2 uptake.